1. Field of the Invention
The invention relates to a method for operating a device for the dosed supply of a liquid.
2. Related Art
A device of this type may also be referred to as delivery device and/or as dosing device. A device of this type may be used, for example, in a motor vehicle in order to feed a liquid additive for exhaust-gas purification to an exhaust-gas treatment device of the motor vehicle. In exhaust-gas treatment devices requiring a liquid additive for operation, the method of selective catalytic reduction (SCR), for example, is implemented. In this method, nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of a reducing agent. Ammonia is commonly used as reducing agent. Ammonia is normally stored in motor vehicles not directly but in the form of a reducing agent precursor solution, which is stored as a liquid in a tank and which is supplied by way of a corresponding device, which may be operated in accordance with the method described here. A liquid that is particularly commonly used in this context is urea-water solution which, for the purposes of exhaust-gas purification, is available under the trade name AdBlue® with a urea content of 32.5%.
For successful exhaust-gas purification, accurately dosed metering of the liquid into an exhaust-gas treatment device is necessary, that is to say the amount of liquid metered corresponds as accurately as possible to a requirement in the exhaust-gas treatment device. Furthermore, a device for the dosed supply of a liquid should be producible and operable as inexpensively as possible, and should as far as possible not be damaged by liquid freezing within the device, because the described liquids can freeze at low temperatures. In motor vehicles, low temperatures arise for example during relatively long standstill phases.
For the delivery of such liquids, devices which have a non-branched delivery line from a tank to an injector at an exhaust-gas treatment device have proven to be particularly advantageous. Such devices for providing a supply typically do not have a return line which permits circulation of liquid through the device and back into the tank. A return line of this type was provided because it was possible for air bubbles in the device to be easily and reliably removed via the return line. However, a return line of this type entails increased cost expenditure.
The dosing (i.e., metering)of the liquid by way of the device is preferably performed with the aid of an injector (dosing valve), which can be opened and closed in electronically actuated fashion. The amount of liquid dispensed by the device is in this case typically proportional to the opening time of the injector. For this purpose, the pressure of the liquid prevailing at the injector must correspond as accurately as possible to a target pressure, because the amount of liquid dosed by an injector during a predefined opening time exhibits a high dependency on the pressure of the liquid at the injector.
Between multiple individual dosing processes of the injector, it is desirable for the pressure of the liquid in a pressure line section between a pump and the injector to be kept substantially constant in order that the injector can dose liquid in the desired manner immediately when a dosing demand is placed on the injector.
Devices for the supply of a liquid typically have at least one pump for the delivery of the liquid and for maintaining the pressure at the injector. However, the pumps used for such devices typically have the disadvantage that a backflow of liquid can occur through the pump. As a result of this backflow, the pressure at the injector is falsified, and thus the dosing accuracy is reduced.